1. Field of the Invention
The invention generally relates to cutting inserts for use in drilling tools and, more particularly, to cutting inserts for use in drilling tools for drilling a stack of plates. The invention also relates to a device for drilling a stack consisting of a plurality of unconnected plates.
2. Background Information
High-performance drilling tools can often be fitted with interchangeable cutting inserts on which the chip-removing cutting edges are formed. The cutting edges, which are exposed to high wear during the drilling operation, can therefore simply be interchanged, while the drilling tool can continue to be used. For retaining the one or each cutting insert in a fixed position, the drilling tool is provided at a drilling head with a corresponding number of cutting insert receptacles, each of which is formed complementary to the outer contour of the respectively associated cutting insert, such that the cutting insert can be inserted into the receptacle in an accurately fitting manner.
In order to be especially useful, such a cutting insert is often designed as a multi-edge indexable insert, which can be fastened in the receptacle in various orientations, in each case one of a plurality of main lips being in the intended working position in each orientation provided.
A special type of such a multi-edge indexable insert is the “trigon cutting insert”, which is described, for example, in U.S. Pat. No. 5,354,156 A. In a plan view of its end faces, such a trigon cutting insert has the shape of a hexagon having three equal “acute” corner angles α (where 60°<α<20°) and three equal obtuse corner angles β (where 180°>β>120°), wherein obtuse and acute corner angles alternate with one another. In a typical form, the acute corner angles have, for example, a magnitude of 84°, whereas the obtuse corner angles have a magnitude of 156°. A conventional trigon cutting insert has three main lips, of which one in each case extends between two acute corner angles. Each main lip extends in this case over one of the obtuse corner angles and is subdivided by the latter into two sections set relative to one another.
A corresponding trigon cutting insert receptacle in the drilling tool is normally formed by a recess which has a substantially axially-tangentially oriented bearing surface which is partly bordered by side walls. In this case, two rear side walls, i.e. side walls facing the shank end, enclose an angle corresponding to the acute angle of the trigon cutting insert. Here, the receptacle is normally oriented in such a way that an angle bisector of this acute angle is either oriented parallel to the tool axis of the drilling tool or is inclined slightly relative to the tool axis, such that the angle bisector runs slightly outward toward the tip of the drilling head. In a normal standard drilling tool, the inclination of the or each cutting insert receptacle (i.e. the setting angle of the angle bisector relative to the tool axis) is in this case always smaller than half the acute angle of the trigon cutting insert. This orientation of the cutting insert receptacle orients the inserted trigon cutting insert, in which the two bisectors of the active main lip are set in opposition with respect to a radial plane perpendicular to the tool axis. Whereas the externally cutting bisector (and therefore the bisector further away from the tool axis) runs radially outward away from the tip of the drilling head, the internally cutting bisector (and therefore the bisector situated closer to the tool axis) runs radially outward toward the tip of the drilling head.
During the drilling operation (and thus during rotation about the tool axis), the two bisectors of the active main lip therefore always produce an annular groove at the bottom of the drill hole, the base line or apex line of which is formed by the obtuse angle between the two bisectors.
During the drilling of workpieces from the solid, this design of the drilling tool and of the cutting inserts is advantageous for a balanced distribution of the cutting forces. However, this design is problematical when drilling unconnected stacks of plates. This is because, in this application, every time the active main lip reaches a plate limit with the obtuse corner angle which lies between the bisectors, an annular material region is cut free in the region of the internally cutting bisector, and this annular material region rotates with the drilling tool and therefore at least makes the further drilling operation more difficult, if not even impossible. In addition, the material ring cut free can also jam the drilling tool, which in the worst case can lead to the destruction of the trigon cutting insert or even of the entire drilling tool.
To avoid this problem, it is known from U.S. Pat. No. 5,269,618 A to increase the inclination of the trigon cutting insert receptacle in such a way that the inner bisector of the active cutting edge of the inserted trigon cutting insert runs at least at right angles to the tool axis or even runs away (in turn radially outwardly) from the front side of the drilling tool.
However, since such a design, as mentioned, is disadvantageous for the distribution of the cutting forces, a special tool is created by the inclination of the trigon cutting insert receptacles according to U.S. Pat. No. 5,269,618 A, the advantages of which only prevail when drilling loose stacks of plates. In view of the relatively low frequency of this application, such a special tool represents a disproportionately high investment in particular for smaller firms.
Accordingly, there exists a need for a device which on the one hand can be used advantageously for drilling unconnected stacks of plates, but on the other hand can also be flexibly used or re-set for other purposes.